Jacket
The paper cover sometimes called the "dust cover" of a hardbound book.

Job Number
A number assigned to a printing project used for record keeping and job tracking. Also used to retrieve old jobs for reprints or reworking by customer.

Jogger
Vibrating, sloping platform that evens up the edges of stacks of paper.

Justify
The alignment of text along a margin or both margins. This is achieved by adjusting the spacing between the words and characters as necessary so that each line of text finishes at the same point.